Aerosol - a colloid of fine solid particles

An aerosol is a colloid of fine solid particles or liquid droplets, in air or another gas.Aerosols can be natural or not. Examples of natural aerosols are fog, forest exudates and geyser steam. Examples of artificial aerosols are haze, dust, particulate air pollutants and smoke. The liquid or solid particles have diameter mostly smaller than 1 μm or so; larger particles with a significant settling speed make the mixture a suspension, but the distinction is not clear-cut. In general conversation, aerosol usually refers to an aerosol spray that delivers a consumer product from a can or similar container. Other technological applications of aerosols include dispersal of pesticides, medical treatment of respiratory illnesses, and combustion technology. Diseases can also spread by means of small droplets in the breath, also called aerosols.

Aerosol science covers generation and removal of aerosols, technological application of aerosols, effects of aerosols on the environment and people, and a wide variety of other topicAn aerosol is defined as a colloidal system of solid or liquid particles in a gas. An aerosol includes both the particles and the suspending gas, which is usually air. Frederick G. Donnan presumably first used the term aerosol during World War I to describe an aero-solution, clouds of microscopic particles in air. This term developed analogously to the term hydrosol, a colloid system with water as the dispersing medium.Primary aerosols contain particles introduced directly into the gas; secondary aerosols form through gas-to-particle conversion.

Various types of aerosol, classified according to physical form and how they were generated, include dust, fume, mist, smoke and fog.

There are several measures of aerosol concentration. Environmental science and health often uses the mass concentration (M), defined as the mass of particulate matter per unit volume with units such as μg/m3. Also commonly used is the number concentration (N), the number of particles per unit volume with units such as number/m3 or number/cm3

The size of particles has a major influence on their properties, and the aerosol particle radius or diameter (dp) is a key property used to characterise aerosols.

Aerosols vary in their dispersity. A monodisperse aerosol, producible in the laboratory, contains particles of uniform size. Most aerosols, however, as polydisperse colloidal systems, exhibit a range of particle sizes.Liquid droplets are almost always nearly spherical, but scientists use an equivalent diameter to characterize the properities of various shapes of solid particles, some very irregular. The equivalent diameter is the diameter of a spherical particle with the same value of some physical property as the irregular particle.The equivalent volume diameter (de) is defined as the diameter of a sphere of the same volume as that of the irregular particle.Also commonly used is the aerodynamic diameter.sFor a monodisperse aerosol, a single number—the particle diameter—suffices to describe the size of the particles. However, more complicated particle-size distributions describe the sizes of the particles in a polydisperse aerosol. This distribution defines the relative amounts of particles, sorted according to size. One approach to defining the particle size distribution uses a list of the sizes of every particle in a sample. However, this approach proves tedious to ascertain in aerosols with millions of particles and awkward to use. Another approach splits the complete size range into intervals and finds the number (or proportion) of particles in each interval. One then can visualize these data in a histogram with the area of each bar representing the proportion of particles in that size bin, usually normalised by dividing the number of particles in a bin by the width of the interval so that the area of each bar is proportionate to the number of particles in the size range that it represents. If the width of the bins tends to zero, one gets the frequency function:

 \mathrm{d}f = f(d_p) \,\mathrm{d}d_p

where

 d_p is the diameter of the particles

 \,\mathrm{d}f  is the fraction of particles having diameters between d_p and d_p + \mathrm{d}d_p

f(d_p) is the frequency function

Therefore, the area under the frequency curve between two sizes a and b represents the total fraction of the particles in that size range:

 f_{ab}=\int_a^b f(d_p) \,\mathrm{d}d_p

It can also be formulated in terms of the total number density N:

 dN = N(d_p) \,\mathrm{d}d_p

Assuming spherical aerosol particles, the aerosol surface area per unit volume (S) is given by the second moment:

 S=  \pi/2 \int_0^\infty N(d_p)d_p^2 \,\mathrm{d}d_p

And the third moment gives the total volume concentration (V) of the particles:[14]

 V=  \pi/6 \int_0^\infty N(d_p)d_p^3 \,\mathrm{d}d_p

One also usefully can approximate the particle size distribution using a mathematical function. The normal distribution usually does not suitably describe particle size distributions in aerosols because of the skewness associated a long tail of larger particles. Also for a quantity that varies over a large range, as many aerosol sizes do, the width of the distribution implies negative particles sizes, clearly not physically realistic. However, the normal distribution can be suitable for some aerosols, such as test aerosols, certain pollen grains and spores.

A more widely chosen log-normal distribution gives the number frequency as:

 \mathrm{d}f = \frac{1}{\sigma\sqrt{2\pi}} e^{-\frac{(d_p - \bar{d_p})^2}{2 \sigma^2} }\mathrm{d}d_p

where:

 \sigma is the standard deviation of the size distribution and

 \bar{d_p} is the arithmetic mean diameter.

The log-normal distribution has no negative values, can cover a wide range of values, and fits many observed size distributions reasonably well.

Other distributions sometimes used to characterise particle size include: the Rosin-Rammler distribution, applied to coarsely dispersed dusts and sprays; the Nukiyama-Tanasawa distribution, for sprays of extremely broad size ranges; the power function distribution, occasionally applied to atmospheric aerosols; the exponential distribution, applied to powdered materials; and for cloud droplets, the Khrgian-Mazin distribution.